The thing is, though, if you put such a substance in a dark vacuum, what's to stop it cooling to absolute zero? The process of electromagnetic radiation is the

The thing is, though, if you put such a substance in a dark vacuum, what's to stop it cooling to absolute zero? The process of electromagnetic radiation is the inverse of electromagnetic absorption, and so what will happen here is that the sample will reach a state of thermal equilibrium with the walls of its container (or the 2.7K cosmic background radiation if we're in deep space) through the exchange of infra-red radiation. If it went to a lower temperature, we could create a perpetual motion machine by putting a thermocouple between the painted object and its container. It is possible to cool some things with radiation, but that requires creating a state that is not in thermal equilibrium, which is not plausible for paint on a wall. https://en.wikipedia.org/wiki/Laser_cooling https://en.wikipedia.org/wiki/Laser_cooling